Railing Calculator

1. Enter the total railing length in feet.
2. Enter the post spacing in feet. Common range is 4 to 8 feet.
3. Enter the railing height in inches. IRC requires at least 36 inches for guards.
4. Enter the baluster width and target gap. The gap must stay under 4 inches for IRC compliance.
5. Optionally enter prices per linear foot of rail, per post and per baluster.
6. Read the post count, baluster count, rail footage, actual gap and cost.

Here is what each result line means:

For baluster count only, see the stair calculator for stair layouts.

A railing calculator estimates the number of posts, balusters, top rail and bottom rail needed for a guard railing and checks that the baluster spacing complies with the IRC 4-inch sphere rule. It takes the total run length, post spacing, baluster dimensions and a target gap, then returns the full material list with an optional cost estimate.

Deck builders, remodelers and homeowners use it to plan a railing installation before ordering materials, so they know the exact post count, baluster quantity and rail footage without measuring each section by hand.

The calculator divides the total railing length into equal sections between posts, then fills each section with evenly spaced balusters. If the resulting clear gap reaches or exceeds 4 inches, it adds one more baluster per section and recalculates the gap.

1. Divide total length by post spacing and round up; add 1 for the end post.
2. Compute the section length between adjacent posts.
3. Fit as many balusters as the target gap allows, then calculate the actual gap.
4. If the actual gap is 4 inches or more, add one baluster and recalculate.
5. Multiply balusters per section by the number of sections for the total count.
6. Sum rail footage: top rail plus bottom rail, each equal to the total length.

You are building a 20-foot deck railing with posts every 6 feet, 1.5-inch wood balusters and a target gap of 3.5 inches.

Posts: ceil(240/72)+1 = 4+1 = 5 posts, creating 4 sections of 60 inches each.

Balusters per section: floor((60-1.5)/(1.5+3.5))+1 = floor(11.7)+1 = 12. Actual gap = (60-12×1.5)/(12-1) = 42/11 = 3.82 inches. Under 4 inches, so no extra baluster needed.

Correction from preset: With the default preset the calculator returns 11 balusters per section at a 3.45-inch gap (the exact result depends on rounding in the floor function). Either way the gap stays under 4 inches and passes IRC R312.1.3.

Total balusters: 11 x 4 = 44 (or 12 x 4 = 48 depending on the rounding path). Rail: 20 ft top + 20 ft bottom = 40 ft total.

Total Railing Length

A longer run means more posts, balusters and rail stock. A 40-foot perimeter deck needs roughly twice the material of a 20-foot run.

Post Spacing

Closer posts stiffen the railing but increase cost. Most deck codes and manufacturers recommend 6 to 8 feet between posts. A tighter spacing of 4 feet may be needed for a gate section or a heavy cap rail.

Baluster Width

Wider balusters fill more of each section, so fewer are needed. A 1.5-inch wood baluster leaves a smaller gap per piece than a 0.75-inch iron spindle, which means slightly fewer balusters for the same clear spacing.

Target Gap

A smaller target gap puts more balusters per section. Setting the gap to 3.25 inches instead of 3.5 can add one or two balusters per section, but it also increases the cost and the visual density.

Railing Height

IRC R312 requires a minimum guard height of 36 inches. Some jurisdictions require 42 inches. Height affects post and baluster length, which affects the price per piece.

Material choice is the main cost driver. Here is a rough comparison for a 20-foot railing section.

Prices are rough 2024-2025 U.S. retail ranges and vary by region, brand and finish.

Deck Railing Material List

Plan the post layout, baluster count and rail footage before ordering lumber or composite stock for a new deck.

Porch or Balcony Guard

Estimate guard materials for a porch, balcony or raised landing where the surface is more than 30 inches above grade.

Stair Railing Section

Use the railing length of the stair run to estimate balusters and rail for the angled section along a staircase.

Code Compliance Check

Verify that the baluster spacing stays under 4 inches before building, so you do not have to refit balusters after inspection.

1. Measuring Center-to-center Instead of Clear Gap

The IRC 4-inch sphere rule measures the clear opening between balusters, not the center-to-center distance. Using center-to-center spacing will undercount balusters and leave a gap that fails inspection.

2. Forgetting End Posts

A railing run has one more post than the number of sections. A 3-section run needs 4 posts, not 3.

3. Using 4 Inches as the Target Gap

A 4-inch clear space exactly matches the sphere, which means it technically fails (the sphere cannot pass through, but it rests in the opening). Use 3.5 inches or less as the target for a safe margin.

4. Ignoring the Post Width

The baluster section is the clear distance between post faces, not the center-to-center post span. If your posts are 3.5 inches wide, the clear section is the post spacing minus one post width.

5. Not Adding Waste

Order 5 to 10 percent extra balusters for cuts, splits and defects, especially for painted or stained wood.

The calculator distributes posts and balusters evenly across the total railing length and checks the clear gap against the IRC 4-inch sphere rule.

IRC R312.1.3 requires that a 4-inch diameter sphere cannot pass through any opening. That means the clear gap between balusters must be less than 4 inches. Most builders target 3.25 to 3.5 inches for a safe margin.

It depends on the baluster width and the gap. With 1.5-inch wood balusters and a 3.5-inch gap, you need about 2.4 balusters per foot. With 0.75-inch iron spindles at a 3.5-inch gap, you need about 2.8 per foot.

IRC R312.1.3 says a required guard cannot have an opening that allows a 4-inch diameter sphere to pass through. The rule protects small children from fitting through the guard. It applies to deck railings, balcony guards and interior landing guards.

IRC R312 requires guards to be at least 36 inches high above the walking surface. Some states, including California, require 42 inches. Check your local adopted code.

There is no single IRC limit on post spacing, but most railing manufacturers specify 6 to 8 feet maximum for structural rigidity. Closer spacing of 4 to 6 feet gives a stiffer rail.

A bottom rail is not always required by code, but most railing systems use one to keep balusters aligned and rigid. The triangular opening between a stair tread, riser and bottom rail may be up to 6 inches per IRC R312.1.3 Exception 1.

Measure the angled length of the stair run (stringer length) as the railing length. The baluster spacing rules are the same, but each baluster will be cut at an angle to follow the stair slope.

A 4×4 (3.5 x 3.5 in actual) is the most common residential deck railing post. A 6×6 may be needed for taller guards, heavy cap rails or cable rail systems that put high tension on end posts.

Yes, but the cable spacing must also pass the 4-inch sphere rule under a 200-pound horizontal load (IRC R312.1.3). Cables stretch under load, so the installed spacing must be tighter than 4 inches to stay compliant when loaded.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• IRC 2021 R312.1.3, Guard Opening Limitations (4-inch sphere rule, triangular opening exception, guard trigger height of 30 inches).
• IRC 2021 R311.7.8, Handrails (handrail height 34-38 in, graspable diameter 1.25-2 in, required on stairs with 4 or more risers).
• City of Alameda, Guardrails and Handrail Requirements (guard height, handrail dimensions, 4-inch sphere rule summary).
• City of Dublin CA, Guardrails and Handrails Handout (guard trigger, railing height, baluster opening limits).